1. Field of the Invention
The present invention relates to a method of optically scanning a scanned face which can be used in a laser beam printer or digital copying machine.
2. Description of the Related Art
There has been widely known a method of optical scanning which a comprises the steps of deflecting at an equi-angular velocity a laser beam emitted from a laser emitting device, condensing the deflected laser beam by means of an image-forming optical system including f-.theta. lenses to form a light spot on the scanned face, and scanning optically the scanned face with the formed light spot.
In the conventional optical scanning apparatus in which the above-mentioned optical scanning method has been realized, the laser beam for optical scanning is deflected at a relatively small angle, and thus makes it possible to keep in an excellent condition the f-.theta. characteristic of f-.theta. lenses used in the image-forming optical system. For this reason, the time t allotted for scanning one picture element in optical scanning has been determined so as to satisfy the following relation: EQU t=L/(f.multidot.N.multidot..omega.) (1)
where f is the focal length of the image-forming optical system with respect to the main scanning direction, L the effective scan width, N the number of picture elements within the effective scan width L and .omega. the angular velocity of the deflection of deflected laser beam.
In recent years, however, there have been a strong demand for decrease in its size and cost in the art of optical scanning apparatuses, resulting in necessity of widening field angle of an image-forming optical system and reducing the number of lenses constituting the image-forming optical system. This requirement gives rise to difficulty for producing an image-forming optical system having an excellent f-.theta. characteristic.
In a case where an image-forming optical system does not have a sufficiently good f-.theta. characteristic, the velocity of light spot moving on the scanned face varies depending upon the optically scanned positions, and this makes noticeable magnification error that an image written-in by the optical scanning is expanded and contracted in accordance with scanning positions in the main scanning direction, and that the entire image differs from the recording dimension set in its design.
As a method of correcting such a magnification error, there has been known a method in which the optical scanning is performed while varying the time t for scanning one picture element in accordance with positions of the light spot in optical scanning (Japanese Patent Application Laying-open No. 32769/1987), however the realization of this method needs a complicated circuit device, and does not satisfy the demand of decreasing the cost of the optical scanning apparatus.
In addition, for the purpose of decreasing the cost of an optical scanning apparatus, plastic optical elements are being introduced into the optical scanning apparatus, however this application of plastic optical elements to an optical scanning apparatus will further increase the magnification error mentioned above due to variations in characteristic of plastic optical elements caused with change in temperature.